(a) Field of the Invention
The present invention relates to a formed permanent magnet to be used with a speed sensor for an automobile and the like.
(b) Description of the Prior Art
For a speed sensor for an automobile, rotation sensor for a motor for a tape recorder, etc., it is required to obtain a large number of output pulses within one rotation. Besides, high accuracy is required for the output pulses, i.e., the amplitude, pulse interval, etc. of the output pulses should be uniform.
Known formed permanent magnet used with this kind of sensors are constructed, for example as shown in FIG. 1 and FIG. 2, by providing multiple magnetic poles, i.e., N poles and S poles, alternately on the peripheral surface of a disk-shaped base. To form the above-mentioned formed permanent magnet, a disk-shaped base of magnetic material is magnetized by using a magnetizer which is composed as shown in FIG. 3. That is, the magnetizer is constructed by forming yokes 2 in a number equal to the number of required magnetic poles and by winding coils 3 onto those yokes 2. The disk-shaped base 1 of magnetic material is magnetized by energizing the coils 3 by an electric current of 1 kA to 100 kA so that the N poles and S poles are formed alternately as shown in the figure.
When the disk-shaped base of magnetic material is magnetized by the above-mentioned known magnetizer, the magnetizing result is subjected to the direct influence of pitch error of magnetizing yokes 2, irregularity in winding of coils 3, etc. and, consequently, dispersion occurs in the distance between magnetic poles of the formed permanent magnet. Therefore, when the formed permanent magnet obtained as above is used in a sensor, dispersion occurs in the output pulses from the sensor and, as a result, the accuracy of the sensor becomes unfavourable.